1. Field of the Invention
The present invention relates to a light-emitting apparatus.
2. Description of the Related Art
In recent years, as a backlight of a liquid crystal panel or as an illumination device, a light-emitting apparatus is employed which uses a light-emitting element such as an LED (light-emitting diode) or the like. Related techniques are disclosed in Japanese Patent Application Laid Open No. 2006-114324, Japanese Patent Application Laid Open No. 2008-300208, Japanese Patent Application Laid Open No. 2006-339298, Japanese Patent Application Laid Open No. 2008-064477, Japanese Patent Application Laid Open No. 2008-258428, and Japanese Patent Application Laid Open No. 2007-158083. FIG. 1 is a circuit diagram showing an example configuration of a light-emitting apparatus investigated by the present inventor. It should be noted that the present applicant has recognized that the configuration and operation of a light-emitting apparatus 1003 described below is outside the scope of conventional techniques. The light-emitting apparatus 1003 includes multiple (n-channel) LED strings 6—l through 6—n, a switching power supply 1004, and a current driving circuit 1008.
Each LED string 6 includes multiple LEDs connected in series. The switching power supply 1004 is configured to step up an input voltage VIN, and to supply a driving voltage VOUT thus stepped up to an anode-side end of each of the multiple LED strings 6_1 through 6—n. 
The current driving circuit 1008 is configured to adjust the luminance level of the LED strings 6 using a combination of an analog diming control method and a burst dimming control method (which will also be referred to as the “PWM dimming control method”). A current source CSi (1≦i≦n) is connected to a cathode-side end of the corresponding LED string 6—i, and is configured to supply a driving current ILED to the LED string 6—i according to the target luminance level. The dimming control method in which the luminance level is adjusted according to the magnitude of the driving current ILED is referred to as the “analog dimming control method”.
A PWM controller 1009 is configured to generate dimming pulse signals PWM1 through PWMn having a duty ratio that corresponds to the target luminance level, so as to intermittently turn on the current sources CS1 through CSn with a duty ratio that corresponds to the dimming pulse signal PWM. With such an arrangement, the driving current ILED flows through the LED strings 6 only during the on-period (on time) TON. Such an arrangement controls the time average of the driving current ILED, thereby adjusting the luminance level.
The switching power supply 1004 includes an output circuit 1102 and a control IC 1100. The output circuit 1102 includes an inductor L1, a switching transistor M1, a rectifier diode D1, and an output capacitor C1. The control IC 1100 is configured to control the on/off duty ratio of the switching transistor M1 so as to adjust the driving voltage VOUT.
The control IC 1100 is configured to stabilize the driving voltage VOUT such that the voltage between both terminals of the current source CS, i.e., the voltage VLED (which will be referred to as the “detection voltage”) that develops at a cathode-side end of the LED string 6, matches a predetermined reference voltage VREF.
An error amplifier 22 is configured to amplify the difference between the reference voltage VREF and the lowest from among the multiple channels of detection voltages VLED1 through VLEDn (which will simply be referred to as the “detection voltage VLED”), so as to generate a feedback voltage VFB. The error amplifier 22 includes a transconductance amplifier (gm amplifier) 21, a feedback switch SW1, a phase compensation resistor RFB, and a capacitor CFB. The feedback switch SW1 is turned on when the dimming pulse signal PWM indicates the turning-on time. The gm amplifier 21 is configured to generate a current that corresponds to the difference between the detection voltage VLED and the reference voltage VREF. The capacitor CFB is charged and discharged using the current thus generated, thereby generating the feedback voltage VFB at the feedback (FB) terminal.
A pulse modulator 20 is configured to generate a pulse signal SPWM having a duty ratio that corresponds to the feedback voltage VFB. A driver 28 is configured to drive a switching transistor M1 according to the pulse signal SPWM. In the burst dimming control operation, the driver 28 is configured to perform switching of the switching transistor M1 only during the on-period TON, and to suspend the switching of the switching transistor M1 during the off-period TOFF.